Interpreting Interpretations: An Analysis of the Bike Accident Tracker

About two weeks ago, Zusha Elinson of The Bay Citizen published an article detailing the “Bike Accident Tracker” that his team developed to map and analyze accidents involving cyclists in San Francisco.   There are a number of really incredible aspects of this data app as well as a few questions yet to be answered regarding its utility.

First, the good stuff.   Harmonizing accident data is an important step in drawing attention to potential problem areas within a city.   The data was supplied by the San Francisco Police Department, and includes collisions reported between January 1, 2009 and November 30, 2010.   Even if some accidents go unreported, the 1,147 accidents that were reported for this period, when mapped, begin to depict patterns.   For example, the Mission has the highest number of accidents per year, more than twice the number of collisions than the Financial District.

People also have an opportunity to add unreported accidents to the map, but smartly, these entries are all reviewed before being posted on the map, and user-submitted data is clearly distinguished from SFPD data.

Bike Accident Tracker
Image Credit: The Bay Citizen

The report also goes a bit deeper than simply mapping the data.   The leading causes of accidents involving cyclists are included in the analysis, as is information regarding who was at fault (cyclist vs. motorist).   Cyclists are reported as being at fault 50% of the time, most often for speeding (yes, cyclists speed, too) and disobeying red lights and stop signs.   Motorists are also guilty of speeding, failing to signal, and dooring.

This is all great information to track and scrutinize.   But, without ridership information to analyze in conjunction with the accident data, how useful is the Bicycle Accident Tracker?   The San Francisco Municipal Transportation Agency does collect statistics on ridership, but its counts are based on a single day of manual counting in August.   While there are some useful trends noted in the SFMTA annual report, such as identifying a 58% increase in observed cyclists since 2006, it is difficult to draw conclusions from one day of counting each year.


In Arlington, Virginia, bike counts are performed over a three-day period every quarter.   In other cities, including Portland, automated counters have been installed to measure traffic.   Of course, more frequent counts and automated systems are more expensive than annual counts, but the National Bicycle and Pedestrian Documentation Project correctly states, “Without accurate and consistent demand and usable figures, it is difficult to measure the positive benefits of investments in these modes, especially when compared to the other transportation modes such as the private automobile.”   Similarly, it is difficult to properly assess problem areas without accurate ridership data.

According to Elinson, “our hope is that the tracker is used on many different levels: by policy makers and transportation planners when making decisions about street design and transportation policies and by cyclists and drivers to understand dangerous spots in the city and how to make them safer.”   I hope so, too.   I also hope that, whether it is Elinson’s team, the SFMTA or a third party, more comprehensive data about ridership can be collected and reviewed in conjunction with the accident tracker.


The Mission has more than double the number of annual accidents as the Financial District, but is the Mission District a more or less safe place to ride a bike?   If the Mission has ten times the number of cyclists annually but only twice as many accidents involving cyclists, then what do the accident statistics really indicate?

The Bike Accident Tracker is an important tool.   The tracker can “help move everyone past the overly-simplified arguments about cars vs. bikes and spur discussions on bike safety that are based on data instead of preconceived biases,” says Elinson.   And he’s right–let’s use the data, including the information about what causes accidents, to be more conscientious cyclists.

It is vital that we make the best use of the data that we have regarding accident locations and causes of accidents, but it is equally important to explore ways to improve the usefulness of the tracker by collecting and overlaying ridership statistics.   When advocates take this tool to the powers that be in San Francisco or in other cities, then they can not only point out problem areas, but also they can demonstrate demand, context and criticality.

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7 thoughts on “Interpreting Interpretations: An Analysis of the Bike Accident Tracker”

  1. The SFMTA has actually been installing digital bike counters embedded into bike paths all across the city, in order to start gathering realtime daily bike traffic statistics.

    This project is fairly new, so I’m not sure if any data from it has been made available.

  2. Stacey Moses says:

    That’s good news. Interesting that the new counters are not listed as recent or upcoming projects on the SFMTA site, but Streetsblog San Francisco has information about where the counters are being installed.

    Manual counts definitely serve a purpose- you can’t get data on demographics, helmet usage, etc. from an automatic counter- but for the purposes of the accident tracker, having this ridership data to overlay would be great. Does anyone know when/where the info will be posted?

  3. MB says:

    Arlington’s quarterly manual counts have been replaced by an annual one, I believe, as it now has three automated counters up and running. As you note, this does mean less demographic info.

  4. Stacey Moses says:

    Ah, thanks Mark. Although BikeArlington’s site links to the National Bicycle and Pedestrian Documentation Project, which lists quarterly dates through 2012, it looks like Arlington is indeed moving to September counts only.

  5. Harrison says:

    I find Mission Street to be a very unsavory area and try and avoid it at all costs. Probably more bikers there than others parts so maybe the accidents are not statistically significant?

    1. Ted Johnson says:

      Bingo. My biggest complaint about the accident tracker in SF, is that the Bay Citizen designated certain streets and areas as “most dangerous” based simply on a numeric count of accidents. They don’t yet have the traffic data, so they are way ahead of themselves–and maybe a bit irresponsible–when they say, “most dangerous.”

  6. Harrison says:

    Agreed. Way more traffic in the Mission than in most areas of SF I know of except major streets such as Fell, Oak, and Lombard. But Mission Street itself stretches for miles and is 2 lanes in each direction. The accident rates by themselves seem high but I don’t think they are out of the ordinary when compared to the traffic.

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